Design of application that allows the communication between browser and hardware

In this scenario. I have a web application, that runs in the browser, and now I want to access the scanner of the local desktop.

As far as i can see, the only option is installing some type of local agent in each computer, through java or an agent app (.exe).

Example: scanner.js

My question is just about the logic, not code.

Ok, I have a website. Now how the website can connect and communicate with the agent? How can i define the communication port?

What is the concept design of this type of process between an agent that have access to hardware and the local browser?